The stratotype of the
Orasoja Member (Tremadocian, Northeast Estonia): lithology, mineralogy, and
biostratigraphy; 135–154

Heljo Heinsalu, Dimitri Kaljo, Tiia Kurvits,
and Viive Viira

Abstract. The
Orasoja Member constitutes in Northeast Estonia the upper part of the Kallavere
Formation and forms some kind of transition into the Türisalu Formation. The
member belongs to the Lower Ordovician part (Karepa Substage) of the Pakerort
Stage. The Orasoja Member is composed of alternating beds of light quartzose
siltstone and black shale. The share of shale is cyclically variable, but
mainly siltstone is prevailing. Mineralogically quartz is dominant, K-feld­spar (up to 15%) forming a minor component of the rock.
Mineral and grain size composition allows us to distinguish two fairly distinct
cycles in the Orasoja Member, which are in correlation with changes in conodont
associations.Three conodont biozones are distinguished in the
section: Cordylodus proavus, C. lindstromi, and C. angulatus. The first appearance of Iapetognathus sp. in the Rannu Member is interpreted here as
indicating the beginning of the Ordovician System. Graptolite occurrences give
evidence of the presence of the Rhabdinopora
flabelliformis socialis and R. f. anglica subzones.
Earlier dendroids are missing, but this is mainly attributed to a taphonomical
bias.

Abstract. Clastic dikes of sedimentary origin are exotic, but
still remarkable minor geological structures in the Lower Palaeozoic sequence ofnorthern Baltoscandia. They often carry essential information
about the processes and environments that governed during sedimentary breaks.
The first up to 10 cm wide clastic dikes in the Devonian sequence of
Estonia were found in sandstones of the Gauja Formation in Piusa and Tabina
glass sand quarries. Field observations, lithological, mineralogical, and
geochemical studies of dikes revealed that these dikes were formed during a
short sedimentary break at the end of the Middle Devonian. Eluvial sediments
that were partly cemented in tropical arid conditions were the probable source
of sandy matrix of dikes and abundant clasts embedded in it. Gravitational
failure and temporary water stream processes were the ways of mass transport
into the fractures. However, the amount of water was not large enough to fill
the fractures and to cause any kind of stratification during the
within-fracture sedimentation.

Local
uplift of the area and extensional deformation of rocks with the formation of
open fractures in Middle Devonian layers suggest a tectonic phase in the
vicinity of the regional Liepaja–Riga–Pskov zone of
tectonic dislocations. This regional zone was formed due to the Late Caledonian
compression and was afterwards recurrently activated. Opening of the fractures
was immediately followed by filling. At that
moment the levels observable in the present sandstone quarries must have been
located more than 20–30 m above the groundwater level. Subsequent
subsidence of the area caused preferable vertical circulation of groundwater
along the clastic dikes. In comparison with the surrounding rocks goethite-rich
fluids have caused stronger cementation of clastic dikes during the post-dike
history. Also some other minor diagenetic alternation processes took place in
dikes.

Key words: fracturing, clastic dike, sandstones, Devonian, Estonia.

Hydrogeological model of Estonia
and its applications; 179–192

Leo Vallner

Abstract. The hydrogeological model constructed using the code Visual
MODFLOW covers the whole territory of Estonia, the surrounding coastal sea,
Lake Peipsi, and border districts of Russian Federation and Latvia, all
together 88 032 km2. The 13 model layers include all main
aquifers and aquitards from ground surface to as low as the impermeable part of
the crystalline basement. Three-dimensional distribution of groundwater heads,
flow directions, velocities, and rates as well as transport characteristics can
be simulated by the model. Detailed basinwide or local groundwater budgets can
be completed.